Method for making stressed lightweight concrete products



Nov. 9, 1965 o. KJELL-BERGER 3,217,075

METHOD FOR MAKING STRESSED LIGHTWEIGHT CONCRETE PRODUCTS Filed Sept. 6,1962 MOISTURE (PERCENT) I l I 0.? 0.2 0.36.405 0.6 0.8 L0 LL25 L6 2.02.5 3J5 4 s SHRINKAGE(MM/METER) INVENT OR OLOF KJELL- BERGER ATTORNEYSUnited States Patent METHOD FOR MAKING STRESSED LIGHT- WEIGHT CONCRETEPRODUCTS Olof Kjell-Berger, Skovde, Sweden, assignor to SkiivdeGasbetong Aktiebolag, Skovde, Sweden, a corporation of Sweden FiledSept. 6, 1962, Ser. No. 221,913 Claims priority, application Sweden,Sept. 14, 1961,

9,148/61; Aug. 17, 1962, 8,975/62 2 Claims. (Cl. 264-228) The presentinvention refers to a method for the production of a re-inforced lightconcrete plate. The invention also refers to a light concrete plate,produced according to said method.

In the production of products, especially plates of light concrete,which have been re-inforced, the prior art shows placing in the mouldbefore the ready-making and before the raising of the light concretemass a re-inforcement crate or re-inforcement mat. As a rule the raisingtakes place by adding to the light concrete mass a gas developmentmeans, for instance aluminum, said metal reacting with the limecontained in the raw material for forming the light concrete therebyforming hydrogen. Thereby the bulk volume of the mass has increased, sothat its upper surface has raised as by dough raising, but there-inforceinent construction has still remained in its initial place inthe mould. In other words, there has been a movement during the raisingbetween the different particles of the concrete forming mass, on the onehand, and the reinforcement on the other hand, thereby displacing theconcrete forming mass upwardly relative to the re-inforcement.

The concrete mass is thin fluid, when it is first poured into the mould,but simultaneously with its raising a thickening or plastification ofthe mass starts, and at least during the later phase of theabovementioned displacement movement between the mass and there-inforcement, the mass is plastic and thickened to such a degree, thatit has no longer the same power of filling out the space. Theconsequence thereof has been, that an empty space is formed above there-inforcement means running in a horizontal direction, and the massthereby only adheres to the lower side portions of the re-inforcementbars, but not to the upper side, where a moon-sickle like space formsbetween the upper side of the re-inforcement bar and the correspondinglower side of the mass. In tests with re-inforced light concrete platesone could immediately observe that this re-inforcing means did not givethe rigidity which was expected according to theoretical calculations.

It appeared that these actions were dependent exclusively on the aboveindicated phenomenon of lack of contact between the concrete formingmass and the re-inforcement irons, especially the upper side of thehorizontally running irons. This lack of contact with the re-inforcingmeans has been observed in light concrete plates, which have beensectioned. One has therefore also tried to compensate for the lack ofstructural strength capacity by increasing the re-inforcement. As anexample by calculations it will be found, that a section area of thelongitudinal re-inforcement irons of 3.1 cm. would be suflicient inorder that a light concrete plate of standard dimensions, said platebeing 3.46 m. long, should be able to stand for an evenly divided loadof 1,800 kg. per longitudinal meter, but in reality this load capacityrequired about three times as much re-inforcement or iron area, i.e.about 9 cm.

It is obvious, that this is a very serious disadvantage. The great ironmass in the re-inforcement makes the production of the plate moreexpensive. The light concrete is very advantageous in its highinsulation power both for sound and heat, but this will be lost to anessential degree as the iron is a good conductor both for sound andheat, and this degree of conduction will increase directly proportionalto the amount of iron. The plate, due to the great amount of iron, willbe heavier and part of its power of loading will therefore be consumedby its increased weight. The work of mounting the structure orpositioning it in construction of a building is also made more difficultdue to the heavier plate which will be more diflicult to handle. If forsome reason the plate should be sectioned, a greater number ofre-inforcement irons or thicker re-inforcement irons must be cutthrough. If holes should be provided for carrying through conductors,then the risk will increase that the drill will hit a re-inforcementiron directly proportional to the increased horizontal area assumed bythe re-inforcement irons.

Many methods for avoiding or at least decreasing the consequences of theabove mentioned phenomenon have been examined. Through experimentation,it has been readily realized that the possibility close at hand ofworking the concrete forming mass during the raising, so that also theupper side of the re-inforcement irons should stick to them will notprovide the desired structure. The mass must move undisturbed, otherwiseits cellular structure will be damaged.

It is known in the prior art with respect to other materials than lightconcrete, i.e. brick work, that smaller units are joined together to abigger unit by connecting the different parts by using re-inforcementirons, which are in some suitable way inserted into the brick workmaterial, usually by means of cement. In the tests mentioned above, wehave tried to use the same method in the production for re-inforcedlight concrete plates, but these tests have not met with success. Tests,which were made immediately after the production of the plates provedthat they were acceptable and that they fully corresponded to theexpectations, but when they were later on used in practice, they did notgive the desired power of loading. In the mounting of the plates in thebuilding we have been able to observe an immediate bending down underload. This has as a rule been so appreciable that the stability andthereby the power of carrying by the plate was almost fully lost.

The tests have therefore been extended to an investigation of reasonsfor loss of load capacity. It was found, that the light concrete, whenit is removed from the autoclave after steam curing, has a content ofmoisture of between 25% and 30%. However, under normal conditions thismoisture is readily evaporated, i.e. the surface moisture, but due tothe light concrete having another highly estimated property, i.e. anutterly low hygroscopicity, the evaporation of the interior moisturetakes place only slowly. When delivered to buildings therefore the lightconcrete had often a remaining moisture, which could amount toapproximately 25 When the piece of light concrete is thereafter mountedand will get into contact with mortar, plaster and so on, the surfacemoisture is re-instated, and the total moisture again increases. Onlythereafter the moisture slowly decreases during the drying out of thebuilding, so that it will after a normal drying period have decreased to4-8%. Further moisture is thereafter as a rule not consumed, because, asmentioned above, the light concrete is strongly non-hygroscopic. Theinvestigations have now shown that during the drying a shrinkage takesplace of the light concrete. This shrinkage decreases with moistureremoval up to 0.5% from its moist state to its dry state or in otherwords up to 5 mm. per longitudinal meter, and the shrinkage isessentially increased at moisture contents below 810%.

The invention is based upon the understanding that the said shrinkagecauses a relaxation of the tension of the re-inforceinent in there-inforcement irons which have been mounted in the moist state of theplate, this is believed to be the reason for the loss of strength in theabove mentioned prior art structure.

According to the present invention, the light concrete is dried outbefore the joining of the light concrete structure or its parts with there-inforcement bars, whereafter the re-inforcement bars are appliedunder a suitable bias tension in the dried out light concrete. This mayper se be rather weak, and during mounting, transportation and so on thelight concrete may thereafter assume moisture and swell, which will onlyincrease the bias tension, be-

cause of shrinkage thereafter due to natural curing the re-inforcementwill still have its initial bias tension left.

The invention can be used successfully on an undivided piece in the formof a plate of light concrete, in which one drills holes or previouslyarranges holes during molding, for inserting the re-inforcement means.However, the invention is not dependent upon the plate being undividedor composed by a plurality of pieces, the reinforcement can be appliedin the plate after it has been dried out, and it is tensioned in such away, that the plate in the building, were it is also subjected to adrying out of eventually added moisture after the re-inforcement isapplied, will remain as stably re-inforced as at the production.

The invention is further described in connection with the attacheddrawing, in which FIG. 1 shows a diagram of the shrinkage of the lightconcrete as a function of the contents of moisture, and FIG. 2 shows are-inforced light concrete plate according to the present invention,partly in section.

In FIG. 1 the horizontal axis indicates in logarithmic division theshrinkage of a light concrete plate in millimeters per meter of lengthdependent upon the content of moisture indicated along the verticalaxis. The content of moisture is indicated along the vertical axis inpercent.

The diagram according to FIG. 1 is based from the assumption that normalmoisture immediately after steam curing of the piece of light concreteis about 30%. It is then evident from the diagram, that when themoisture has decreased to half of normal moisture after steam curing,that means to only minute shrinkage has taken place, i.e. 0.15millimeter per meter. Thereafter, however, the rate and magnitude of theshrinkage rather quickly increases. At a remaining moisture of 5% ashrinkage from the initial state has taken place of 0.4 millimeter permeter, and when the content of moisture has decreased to zero value theshrinkage has increased to 5 millimeters per meter. If the environmentalclimate change takes place changing from dry to moist, the effect willbe the contrary one expanding the block.

FIG. 2 shows a perspective drawing of a light concrete body, which hasbeen re-inforced according to the present invention. The proper lightconcrete body 10 has either already when moulded been provided withchannels or after moulding by drilling or in another suitable way beenprovided with channels, intended to house the re-inforcement irons 11.These re-inforcement irons are inserted through a pressure distributionplate 12 or 13 respectively at each end of the body 10, said plate beingprovided with holes, corresponding to the channels 14 in the lightconcrete body. The ends of the re-inforcement irons 14 are secured bymeans 15 so that a suitable bias tension is obtained to lock there-inforcement iron in position.

As already mentioned above, the light concrete body should be dried outbefore securing the re-inforcement irons so that none or onlyunessential remaining moisture exists, and only thereafter there-inforcement irons 11 should be introduced through the pressure plates12 and 13 and the channels 14 provided in the light concrete body 10.The bias tension in the re-inforcement irons is thereby adapted by meansof the nuts 15 in such a way, that the bias tension will still remain ata given value, after the light concrete has been fully dried out withthe shrinkage following therefrom.

Even if iron is the natural material for a re-inforcement of the kindhere concerned, iron is nevertheless for several different reasons lessdesirable. On the other hand, prior art showed that only iron could beused as material for the re-inforcement with regard for the demand foran economically obtainable material and also for a sufficiently strongmaterial.

Even if iron has certain advantages as a re-inforcement, which are notto be disputed, the disadvantages of using iron may nevertheless inseveral cases be so serious, that it is economical to use anothermaterial instead of it.

The iron used for the re-inforcement as a rule is hot drawn or colddrawn iron with a lower allowable stretching limit between 13 and 20kg./mm This re-inforcement iron takes up practically all of the strain,to which the light concrete plate is subjected, and in order that itshall get the desirable rigidity in many a case a rather essentialquantity or iron is required. Thereby the product will be inconvenientlyheavy. Further it will conduct sound as well as heat to a rather highdegree. Also that iron has a very great tendency of rusting, after ithas been applied as a re-inforcement in concrete, especially lightconcrete. The tendency of the re-inforcement irons to rust has beendecreased by protection by some suitable means, which makes theproduction of the final product more expensive.

A further developement of the present invention is based upon aninvestigation of the suitability of other materials concerned. It hasbeen found that plastic bars, preferably glass-fiber re-inforced bars ofplastics, are preferred over an iron-re-inforcement.

The specific properties of glass-fiber re-inforced plastic bars, arethat the bars have a very high security against breakage when subjectedto drawing as there is no stretching limit. Therefore, one would assumethat the glassfiber re-inforcement plastic bars would, as are-inforcement in light concrete products be too dumb, and would notallow for normal extension procedures before they break, which the ironwill do. It has also been found, that light concrete products, whichhave been re-inforced by means of iron bars, have differentcharacteristics than those which have been re-inforced by glass-fibers.

The advantages, when using glass-fiber re-inforced plastic bars as are-inforcernent in light concrete products, could, according toinvestigations be summarized in the following way:

In mass production the re-inforcement with glass-fiber plastic is notmore expensive per unit of volume of the light concrete product thanwould have been the case with iron re-inforcement. Certainly, the propermaterial in the re-inforcement will be somewhat more expensive, but thisis compensated for by no rust-protection treatment being required, andthat the product will get a longer life time, because it is not attackedby any kind of corrosion. On the other hand, iron practically withoutexception, even if it has been protected against rust in a very goodway, will sooner or later be attacked by rust. Glass-fiber re-inforcedplastic thus is both weather-proof and agingproof.

The fact that there is no stretching limit for glass-fiber re-inforcedplastic makes possible the calculation of the required amount ofre-inforcement material exclusively with respect to the security againstbreakage. Surprisingly it has therefore proved, that one can use asmaller total area of re-inforcement when using glass-fiber reinforcedplastic in the re-inforcement bars than with iron in these bars.

Plastic, a material which forms the main part of glassfiber re-inforcedplastic bars, which should according to the present invention be usedfor re-inforcement in light concrete products, has a very low power ofconducting heat, whereas iron has a high power of conducting heat. Theheat resistance of the thermal conductively value of light concreteproducts, re-inforced according to the present invention, will thereforebe very good.

The property of the glass-fiber re-inforced plastic bars to be ratherdumb from the point of view of extension causes that they are also verybad conductors for sound. A plate of light concrete, which has beenre-inforced according to the present invention, therefore will be a muchbetter sound insulator than the light concrete product reinforced in thetraditional way by means of iron bars.

In this connection it should be mentioned, that the known or traditionalmethod of re-inforcing light concrete products with iron, has been, thatthe iron bars were placed in the product when it was moulded, the lightcon crete product, usually a plate, thereafter being subjected toraising and steam curing. For other kinds of artificial stone plates,preferably of brick work or similar material, it has been proposed todivide the product in a plurality of mechanically different layers,whereafter one has in the upper side of a lower layer and in the lowerside of an upper layer provided channels, in which one has applied there-inforcement irons, and thereafter one has in one way or anothersecured together the two layers, for example by means of cement slurry.This last mentioned method cannot be used with light concrete products.

The glass-fiber reinforced plastic is not normally substituted, per se,in the process for making the concrete plate because then the plasticwould be together with the light concrete product at the hightemperature of between 180 and 200 C., which exists in the autoclaveduring the steam curing. Plastic cannot withstand this temperature,because the plastic will at this temperature be subjected to interioratomic changes, decreasing the rigidity, and the glass-fiber will meltat this temperature, and therefore its re-inforcing activity will getfully lost.

The use of glass-fiber re-inforced plastic in the form of bars thereforeis subject to the method in which the moulded light concrete productafter having been steam cured in its non-re-inforced state, is dried tonone or only unessential remaining moisture, and the re-inforcement isthereafter introduced under a mechanical bias strain so adapted that itwill increase to a suitable value after the light concrete has beenmoisture-conditioned and consequently has expanded.

When it is mentioned above that one should use glassfiber re-inforcedplastic bars as re-inforcement material, it must not be understood thatthese bars must necessarily have such a form as the traditionalre-inforcement irons, that means substantially circular cross-sectionarea, but any extended form of the re-inforcement should be regarded asa bar. These bars are thus mounted into the light concrete productsafter they have been steam cured, in holes or channels, which have beenprovided in ad- Vance, for instance by cutting, drilling or already atthe moulding.

The tensioning preferably takes place by securing the extending ends ofthe glass-fiber re-inforced plastic bar by providing a greatercross-section area than the cross-section area of the hole through whichthe re-inforcement extends. In other words, the thickness of the barshould be at the ends be greater than the thickness of the bar at itsmiddle part. The thickened part could be conically made or it could besplit end formed. One can also provide the said thickened parts of there-inforcement bars at their ends in any conventional manner. It also ispossible, before the chemical curing of the plastic to introduceanchoring means of metal, even of iron, in the plastic at their barends, or when mounting the re-inforcement to provide the plastic barswith split ends which may either be allowed to solidify in the usualmanner or which may be driven apart by means of wedges, i.e. so calledbracing locks.

When manufacturing the glass-fiber re-inforced plastic bars, whichshould form the re-inforcement in the light concrete products, it ispreferable to add plastic to the bundle of glass fibers at the ends ofthe bars after the mounting of the re-inforcement bar into the lightconcrete product, so that one can connect to these ends the anchoringmeans which are suitably placed. One could even place all of the bardirectly in the light concrete material, first drawing in the bundle ofglass fibers in the holes or grooves in which the re-inforcement shouldbe situated and thereafter pouring in the fluid plastic, which includesthe required curing means and also acceleration means, whereafter thisis allowed to solidify in place. In connection with the positioning ofthese plastic bars directly in the light concrete product, one can ofcourse also take care of required anchoring means simultaneously. Forinstance, one can when drilling of holes for the plastic re-inforcementmake the ends of said holes funnel formed, so that the plasticre-inforcement will be finished with cones, formed according to thesefunnels.

It is not necessary to bring the moisture of the light concrete down to0%, when the re-inforcement takes place, because the expansion which thecontained moisture would cause, could be easily compensated for by thebias tensioning of the re-inforcement iron at its mounting. In acorresponding way one can provide the proper reinforcement procedure inan aggregate for, automatically or semi-automatically bias tensioningthe re-inforcement irons by machine to a constant value, and controllingthe effective bias tension by choice of moisture of the plate at thesecuring of the re-inforcement.

It is advantageous to condition the concrete to a very low moisture whenre-inforcing, but this may meet with economical difficulties. It issubstantially for this reason that one can also according to thisinvention effect the re-inforcement at a certainly decreased but notmaximally decreased moisture, and thereby determine the bias tension inthe re-inforcement in such a way. Therefore, even a fully de-moisturizedplate will have a certain amount of bias tension in the re-inforcementirons. If the plate should thereafter during storing, transportation ormounting assume a higher moisture the bias tension will increaseproportional to the expansion caused thereby. The bias tension will, ofcourse, decrease to its normal value, after the plate has been dried tonormal humidity conditions in the building.

The drying of the light concrete plate or the parts forming the lightconcrete plate can take place either by storing in a dry atmosphere orin artificial Way, for example in a drying oven.

What I claim is:

1. A method for making light weight, reinforced concrete structurescomprising the steps of providing longitudinal, through openings in theconcrete structure for the insertion of reinforcing means, curing thestructure with steam under pressure; drying the cured structure untilsubstantially all of the free moisture is removed from the concrete,inserting reinforcing means in said openings, locking the ends of saidreinforcing means to hold said reinforcing means stationary relative tomovement of said structure and placing said structure in a normalhumidity environment, whereby the reinforcing means is subjected to abias tension when the light concrete structure expands under normalhumidity conditions.

2. The method of claim 1 wherein the reinforcing means areglass-fiber-reinforced plastic formed by inserting glass fibers into theopenings and inserting a mould- References Cited by the Examiner UNITEDSTATES PATENTS 10/33 Hutteman et a1.

3/41 Anderegg 264228 8 Muntz 264-228 XR Freyssinet 25118 XR Siegfried264228 Goldfein 264--228 XR ROBERT F. WHITE, Primary Examiner.

ALEXANDER H. BRODMERKEL, Examiner.

1. A METHOD FOR MAKING LIGHT WEIGHT, REINFORCED CONCRETE STRUCTURESCOMPRISING THE STEPS OF PROVIDING LONGITUDINAL, THROUGH OPENINGS IN THECONCRETE STRUCTURE FOR THE INSERTION OF REINFORCING MEANS, CURING THESTRUCTURE WITH STEAM UNDER PRESSURE; DRYING THE CURED STRUCTURE UNTILSUBSTANTIALLY ALL OF THE FREE MOISTURE IS REMOVED FROM THE CONCRETE,INSERTING REINFORCING MEANS IN SAID OPENINGS, LOCKING THE ENDS OF SAIDREINFORCING MEANS TO HOLD SAID REINFORCING MEANS STATIONARY RELATIVE TOMOVEMENT OF SAID STRUCTURE AND PLACING SAID STRUCTURE IN A NORMALHUMIDITY ENVIRONMENT, WHEREBY THE REINFORCING MEANS IS SUBJECTED TO ABIAS TENSION WHEN THE LIGHT CONCRETE STRUCTURE EXPANDS UNDER NORMALHUMIDITY CONDITIONS.